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A: We frequently refer to quasi-resonant power supplies. But what exactly is a quasi-resonant switching power supply? As we all know, the main loss in a switching power supply comes from the switching process of the switching transistor. Since the transistor is not an ideal switching device, the switching process isn't instantaneous; there's always some transition time involved. As illustrated in Figure 1, traditional square wave switching power supplies experience overlapping areas between voltage and current during this transition phase, resulting in losses. As the operating frequency increases, these losses grow, limiting the maximum frequency of the switching power supply. Additionally, the rapid changes in voltage and current during the transition period generate significant switching noise, creating electromagnetic interference (EMI). To address this issue, researchers have been developing low-power soft-switching technologies for over two decades. By incorporating small inductors or capacitors into the circuit, they leverage resonance principles to ensure that the voltage or current across the switch transitions smoothly as a sinusoidal wave. The core concept here is to find ways to enable the switching transistor to perform switching operations at zero voltage (ZVS - Zero Voltage Switching) or zero current (ZCS - Zero Current Switching), thereby eliminating the overlap between voltage and current, and ultimately reducing power consumption.
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While resonant power supplies (Resonant Switching Power Supplies) reduce switching losses, their circuits tend to be more complex. A quasi-resonant mode is commonly employed in flyback switching power supplies. The term "flyback" refers to the fact that the primary-side main power switch and the secondary-side rectifier operate in opposite switching states. When the switch is turned on, the secondary-side rectifier is off, and the flyback converter only stores energy when the primary-side switch is activated. When the switch turns off, the stored energy is released to the load. Thus, the high-frequency transformer serves dual purposes during the switching process: it provides both transformer isolation and acts as an inductor for energy storage. Due to its simplicity and ability to easily produce multiple outputs, the flyback switching power supply is widely used in color televisions. Unlike resonant switching power supplies, where the resonance actively participates throughout the entire energy transformation process (resulting in oscillations greater than 1, producing a sine wave), quasi-resonant modes complete the resonance process only during one phase of the overall power conversion—specifically, during the switching phase (where the waveform remains closer to a square wave). Through resonance, the switching transistor is able to complete the switching operation at zero voltage (or minimum voltage) or zero current, while maintaining the high-efficiency energy transfer capability of square-wave switching power supplies. This approach is therefore referred to as quasi-resonance (quasi-resonant) QR.